1. Field of the Invention
The present invention relates to a bidirectional optical module for the signal optical fiber which includes a wavelength division multiplexed (WDM) filter.
2. Related Prior Arts
The PON (Passive Optical Network) system has been introduced in the network system between the center and the subscribers, which is typically realized in the FTTH (Fiber to the Home) and provides higher speed and larger capacity communication with a reasonable cost. The PON system implements optical couplers in the communication line to draw the fiber into a plurality of users, which is equivalent to a system where the plurality of users shares the single fiber connected to the center station. The PON system may reduce the count of optical fibers pulled out from the center station, thus, realizes the advanced communication service with higher speed with reduced cost comparable to that using the metal cable. Further, the PON system is one of the WDM system where different wavelengths, one is 1.31 μm band and the other is 1.55 or 1.49 μm band, to transmit and receive optical signals within in the single fiber, which may further enhance the communication capacity.
The bi-direction module for the single optical fiber, which is used in such a PON system, has a configuration that the transmission light emitted from a laser diode (LD) is coupled with the optical fiber, while, the receiving light provided from the optical fiber is coupled with the photodiode (PD). Various techniques and apparatuses have been reported to couple the light with the device or the optical fiber.
For instance, one typical example is an optical module that implements a transmitter optical sub-assembly (TOSA) and a receiver optical sub-assembly (ROSA) each having individual package, what is called as a two-package module. This two-package module is quite popular in the field because simple and conventional assembly techniques are applicable. However, this two-package module follows a cost-demerit because it applies two can packages for the TOSA and the ROSA even the packages are popular and available in the market. Accordingly, another type of the bi-directional module has been proposed, in which the single package installs both the LD and the PD, what is called as the one-package module. Japanese Patent Application published as JP-2004-271921A and U.S. Pat. No. 7,093,988, have disclosed such one-package bi-directional module.
The optical module disclosed in JP-2004-271921A provides a beam splitter set in the side of the sub-carrier that has a step. The transmitting light from the LD, which is mounted on the top of the sub-carrier, reaches the lens after the reflection by the beam splitter. While, the receiving light provided from the fiber enters the PD mounted on the step of the sub-carrier after it concentrated by the lens and the beam splitter.
The optical module disclosed in the U.S. Pat. No. 7,093,988 implements a WDM filter inclined by a preset angle with respect to the optical axis of the optical fiber selectively passes the receiving light from the fiber and reflects the transmitting from the LD. Inner cap, by surrounding the PD, isolates the PD from the stray light scattered invading from the outside.
The one-package module mentioned above is able to miniaturize the whole body, but has a weak point to degrade the crosstalk where the transmitting light emitted from the LD is diffusely reflected by the WDM filter to cause the stray and the PD receives this stray light. This is because of the adjacent mounting between the LD and the PD.
The present bi-directional module with the one-package configuration, in which the LD and the PD are adjacently installed, may improve the optical crosstalk performance.